A number of problems in the petroleum industry derive from the viscosity, surface tension, hydrophobicity and density of crude oil. Heavy crude oil in particular, having an API gravity of less than 20 degrees, is difficult to transport due to its viscosity, and is difficult to remove from surfaces to which it has adsorbed, due to its hydrophobicity and immiscibility with water. Extra-heavy crude oil or bitumen, having an API gravity of less than 10 degrees, is heavier than water, so that it can sink to the bottom of a water formation, causing subsurface contamination.
The properties of crude oil contribute to the limitations of oil recovery from traditional oil fields. Conservative estimates suggest that 30% of the technically recoverable oil in U.S. oil fields is inaccessible due to the adsorption of the residual oil to porous geologies. Technologies to unlock the oil in these so-called “dead” wells presently involve the use of hot water injections with expensive surfactants, chemistries that are applied to overcome the hydrophobicity of the adsorbed oil so that it can be mobilized.
The properties of crude oil also contribute to the difficulty of environmental remediation following, for example, an oil spill onto a body of water. The high interfacial tension causes the oil to float on the water and adhere to plants, animals and soil. As the aromatic constituents of the oil evaporate, the heavier residues can sink, contaminating the subsurface structures. Current treatment of spilled oil on water surfaces relies on time-consuming and expensive biological degradation of the oil. Thick, adherent crude oil cause environmental problems in the oil fields as well. Oil deposits attached to vehicles and equipment must be cleansed with jets of hot water and caustics.
The viscosity of heavy crude oil makes the substance difficult and expensive to transport to upgrading facilities. Because of its viscosity, a significant amount of energy is required to pump it through pipelines to a refinery. Furthermore, the viscosity affects the speed at which the heavy crude oil can be pumped, decreasing the overall productivity of an oil field. Exploiting certain oil fields or other oil deposits may be economically unfeasible to develop at present because of the transportation-related costs.
Crude oil, as it is produced, is typically associated with connate water that can form a stable emulsion with the oil in multiple phases, including solid-in-oil dispersions, water-in-oil emulsions, and oil-in-water-in-oil emulsions. Certain hydrocarbon molecules found in heavy crude oils can act as emulsifiers to stabilize the various species of water plus oil emulsions. As an example, asphaltenes and high naphthenic acids, along with submicron sized solid particles such as silica, clay or other minerals, can stabilize emulsions such as water-in-oil emulsions where the heavy crude oil fluid comprises the continuous phase. Asphaltenes are high-molecular weight, complex aromatic ring structures that can also contain oxygen, nitrogen, sulfur or heavy metals. As polar molecules, they tend to bond to charged surfaces, especially clays, leading to formation plugging and oil wetting of formations. Asphaltenes tend to be colloidally dispersed in crude oils, stabilized by oil resins.
Asphaltenes, paraffinic waxes, resins and other high-molecular-weight components of heavy crude exist in a polydisperse balance within the heavy crude fluid. A change in the temperature, pressure or composition can destabilize the polydisperse crude oil. Then the heavy and/or polar fractions can separate from the oil mixture into steric colloids, micelles, a separate liquid phase, and/or into a solid precipitate. The asphaltene micelles can be destabilized during well treatments, e.g., acidizing or condensate treatments, leading to asphaltene precipitation. Asphaltene precipitation causes problems all along the crude oil process. Asphaltene precipitation becomes increasingly problematic when crude oil is processed, transported, or stored at cooler temperatures, because the heavier components of crude oil (e.g., asphaltenes and naphthenic acids) that remain dissolved in the heavy crude under high temperatures and pressures are no longer supported in that state as the conditions change. When the heavy crude oil cools to ambient atmospheric temperatures, these components can precipitate out of the crude oil itself and lodge at the bottom of a storage vessel or tank to form a viscous, tarry sludge. These components also become available as emulsifying agents to sustain water-in-oil emulsions. The emulsion layer has a higher density than light crude, so that it tends to sink to the bottom of storage vessels along with the heavy oil components and associated clay/mineral solids, contributing to the buildup of oil sludge, a thick waste material formed from the various deposits sedimenting out from a crude oil mixture.
As mentioned previously, sludge forms when heavier components of crude oil separate from the liquid hydrocarbon fractions by gravity and sink to the bottom of the vessel. Components of the sludge can include usable hydrocarbons along with the aforesaid entrained water as a water-in-oil emulsion, along with a multitude or organic and inorganic components and contaminants. As the heavier elements in the stored oil continue to migrate to the vessel bottom, the sludge becomes increasingly viscous over time. Any given storage vessel can thus contain a significant amount of sludge, which can diminish storage space for useful crude oil and which can otherwise reduce the efficiency of storage tank operation. Sludge may also require removal if the storage vessel is to be maintained, repaired or inspected.
Many approaches have been proposed for preventing the formation of sludge in oil storage vessels such as oil tanks and oil tankers, and for removing sludges and oily sediments that have formed. In particular, it is desirable to recapture valuable hydrocarbons from the sludge as part of the removal process. The two dominant systems for sludge removal are surfactant-based approaches and solvent-based approaches. In surfactant-based systems, aqueous solutions are used to treat the sludge and coalesce the water droplets emulsified within the oil matrix. The particular surfactant is designed to overwhelm the surface energy that is created by the asphaltene/naphthenic acid molecules and return the aqueous portion to a more-native interfacial tension with organics. Current surfactant additives have been shown effective but have commercial limitations because of either high dosage requirements or ineffective solids interactions. Solvent systems typically use a mixture of known aromatic and aliphatic-based organics to decrease the viscosity of the heavier oil fractions and cause phase separation. Issues of cost and toxicity, however, have been raised with the use of solvent-based approaches.
The development of a technology that can provide emulsion and favorable transport properties while maintaining the ability to demulsify on demand, all under variable conditions of salinity, temperature, pH, etc., remains unmet in the art. Such a technology would have wide reaching impact across the oilfield chemical sector in applications such as those mentioned above, particularly if the material could be inexpensively produced and could be applied to a variety of oil types.
Additional uses for a surfactant technology in the oil industry arise from the problems posed by oil well drilling. When drilling oil or gas wells, a drilling fluid, referred to as a “drilling mud,” is circulated downwardly through a pipe to reach the drill bit, lubricating it and carrying away the cuttings from the drilling process. The clean drilling mud is injected through a series of pipes called the drill string to reach the bit, and then flows back up to the surface in the annular area between the drill string and the inside of the wellbore carrying the cuttings and other particulate matter. The drilling mud can be water-based or oil-based. Oil-based drilling fluids include as their base material any of a number of natural or synthetic oils, including petroleum fractions, synthetic compounds, blends of natural and synthetic oils, along with a variety of performance-enhancing additives. Following drilling, the wellbore annulus must be cleaned to remove drilling fluids, gelled drilling fluid, residual additives from drilling fluids, and the like. One cleaning process can take place before the casing and cementing operations are done, and another cleaning process is done after the casing is installed. The casing must be cleaned to a water-wet condition with no oil sheen. Oil-based drilling fluids, especially synthetic based muds (SBMs), are particularly difficult to remove from the surfaces they contact. These oil-based fluids can form invert emulsions upon contact with water, where the continuous phase is predominantly organic, and the discontinuous phase is aqueous. This emulsion will tenaciously coat any surface that it contacts, leading to oil wetting of borehole surfaces, casing surfaces, and the surfaces of other equipment that it contacts.
Wellbore cleaning can involve the use of a sequence of fluids, each having a specific purpose. In designing the sequence for the cleaning process, formulations are selected that give maximum performance while using minimum amounts of material. Also, the fluids must be chemically and physically compatible, so that an earlier one does not interfere with the function of subsequent ones. Cleaning operations must be conducted carefully, so that the clay components of the drilling mud residue do not come into contact with water, thereby forming a thick paste that adds to the difficulty of removal. There remains a need in the art, however, for a cleaning system that is effective and efficient in removing drilling mud films and residua from wellbore surfaces. This need is exacerbated by the prevalence of SBMs, which produce harder-to-remove films. There is also a need for a cleaning system that requires less fluid volume than those systems presently in use. In addition, there is a need for a cleaning system that does not require or produce hazardous materials.